1. Field of the Invention
The invention relates to a superconducting fiber bundle which comprises a multiplicity of carrier fibers coated with a superconductive layer of a niobium compound selected from the group consisting of niobium oxycarbonitride and niobium carbonitride of the general formula NbC.sub.x N.sub.y O.sub.z wherein x+y+z is equal to or less than 1, said superconductive layer of niobium compound having a fine-grained B1-structure with a mean grain size between 3 and 50 nm, wherein the layer thickness of the superconducting layer is between 100 and 2000 nm.
2. Description of the Prior Art
In the further development of power technology in view of nuclear fusion and superconducting generators, traffic engineering (magnetic suspension railroad), environment engineering (coal desulfurization) and high energy physics, strong-field magnets are required which can be manufactured economically only on the basis of superconductors.
A new promising superconducting material is, for instance, NbC.sub.x N.sub.y O.sub.z (x+y+z less than or equal to 1), which applied to carrier fibers of a fiber bundle (number of fibers is arbitrary), can be used as a fiber conductor. Niobium oxycarbonitride as well as in particular niobium carbonitride in which z=0, are distinguished by high critical temperatures, high critical magnetic fields and high critical current densities. Any suitable material can be used as the carrier fiber material (for instance carbon, boron or steel) which has the necessary mechanical strength. It serves as a high-tensile strength matrix and as a substrate for a chemical gaseous phase deposit method (CVD=chemical vapor deposition), in which niobium is deposited by reaction of NbCl.sub.5 with H.sub.2 in the presence of carbon and nitrogen-containing gases as a thin film. The CVD process is carried out either in a single stage (simultaneous Nb-deposition and carbonitration) or in two stages (Nb-deposition and carbonitration following each other in time).
From German Published Prosecuted Application DE-AS No. 28 56 885, a 2-stage CVD method has become known which is carried out at gas pressures higher than or equal to normal pressure. In principle, the layers that can be produced by this method are still too coarse grained to achieve optimum superconductor properties. The attainable grain sizes of the niobium carbonitride crystals are 50 to 100 nm.
Superconduction parameters such as the upper critical magnetic field strength H.sub.C2 depend, in addition to the usual influences such as composition, degree of order, purity and similar things, particularly on the metallurgical grain structure. A reduction of the grain size is followed, because of the reduction connected therewith of the free path length of the conduction electrons, by an increase of the Ginsburg-Landau parameter k (k=coherence length/magnetic penetration depth) and, since H.sub.C2 is proportional to k.multidot.H.sub.C (H.sub.C : thermodynamic critical magnetic field strength), an increase of the magnetic field H.sub.C2 in the thermodynamic critical field H.sub.C does not increase to the same extent in this grains size reduction. In the case of niobium oxycarbonitride, the thermodynamic properties, particularly the thermodynamic critical field H.sub.C, do not react sensitively to a further reduction as a consequence of a grain size reduction, because of their already small free path length of the conduction electrons, so that in this substance (as also in other B1 structure superconductors) an increase of the magnetic field H.sub.C2 follow from a reduction of the grain size.
It was possible to demonstrate this increase of the critical magnetic field strength H.sub.C2 due to grain size reduction by means of cathode sputtering by which thin niobium oxycarbonitride films were applied to plane carriers (J. R. Gavaler et al, IEEE Transactions on Magnetics, Volume MAG-17, No. 1, January 1981, Pages 573-576). However, it has not been possible so far to coat fibers of a carrier fiber bundle with superconducting niobium oxycarbonitride with grain sizes of less than 50 nm. As was mentioned, only layers, the grain size of which was more than 50 nm, could be produced by CVD methods. The cathode sputtering method on the other hand has a preferred direction, so that the problem of mutual shading of the individual fibers of the carrier fiber bundle arises. Due to this insufficient throwing power, uniform coating of the individual fibers has not been possible so far. Plane superconductors, however, can be used technically only with great reservations because the eddy currents generated in the ribbons by transversal components of the magnetic field never decay and thereby contribute to electrical instabilities in the ribbon conductor. Otherwise, a reduction of the grain size is desirable in view of increasing the critical current density J.sub.C, since the grain boundaries form very effective adhesion centers for the magnetic flux lines.